OAK-593: Segment-based MK

It looks like Felix has trouble with a null return value from addingService(), so let's not do that.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1452228 13f79535-47bb-0310-9956-ffa450edef68
OAK-643: Very high memory usage with 6000 child nodes
- limit cache size by memory using a Weigher

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1452248 13f79535-47bb-0310-9956-ffa450edef68